Liquid in the feed to natural gas liquid (NGL) recovery units varies dramatically with ambient conditions. Traditional stabilizers and recycle compressors can become overwhelmed with the condensables in the feed (ethane, propane, butanes, i-pentane) that is rejected from gasoline stabilizers. This causes unstabilized gasoline to be sent to atmospheric tanks to “weather off” the volatile organic compounds (VOC). This is a loss of product and a safety hazard. Excessively large and expensive compressors increase capital costs dramatically if the compressor is sized to handle the full load of vapor from the stabilizer when ambient conditions cause large amounts of condensation. A side stripper on the stabilizer is much cheaper, more reliable, and less expensive to maintain. Mechanical inlet feed chilling (rich gas conditioning) also produces much more condensation and requires a larger stabilizer overhead compressor. A side stripper and NGL recovery system in the stabilizer keeps the recompression size to a minimum. The goal of the downstream NGL unit is to recover the liquids. By recovering more liquids prior to the NGL unit, capacity of the overall plant is increased.
Natural gas that comes from gas wells and other sources consists primarily of methane and contains varying amounts of heavier hydrocarbons including ethane, propane, normal butane, isobutane, pentanes and even higher molecular weight hydrocarbons. When these heavier hydrocarbons are processed and purified into finished byproducts, all of these are collectively referred to as NGL (natural gas liquids).
NGL recovery units have various sources of feed stock with different compositions. Rich gases such as well head gas, contain some amounts of natural gasoline (mostly pentanes and heavier hydrocarbons with smaller amounts of C6 and C6+ hydrocarbons). These heavier hydrocarbons condense in surge drums that feed GSP (gas subcooled process) plants that range in magnitude from “drips” to “slugs” of liquid. These liquids need to be stabilized to product specifications to be marketable. Compression units that increase the gas pressure to pipeline pressure also induce liquid condensations that also must be stabilized. Additionally, the feed to NGL recovery plants is cooled to condense the liquids prior to entering the NGL plants to increase the capacity of the NGL recovery unit (rich gas conditioning).
Along with the natural gasoline that can be recovered, significant amounts of other NGL material are condensed with the natural gasoline, thus the need for stabilization.
Conventional natural gasoline stabilization is accomplished by depressurizing the liquid and feeding it to a reboiled stripper column. Stabilized, natural gasoline is produced at the tower bottoms while the vapor is recompressed and recycled back to the NGL unit.
Ambient conditions and its effects on the pipeline temperature vary the amount of condensed material and the composition. Feed gas temperature affects the several key components that condense and affect the duty of the recycle compressor—mainly butanes and i-pentane. In winter conditions at 16° to 27° C. (60° to 80° F.) at pipeline pressures, significant amounts of butanes and i-pentane can condense prior to entering the NGL plant. These are the light-key components of natural gasoline stabilizers and have the largest effect on Reid Vapor Pressure (RVP). These components must be reboiled off of the gasoline to make an acceptable product.
If large quantities of propane, butanes, and i-pentane are condensed into the stabilizer feed, they must be rejected via the overhead vapor of the stabilizer, recompressed, and fed to the NGL recovery plant. Condensing these species may be the result of ambient conditions, compression, or inlet chilling. The recycle of these species causes the duty of the recompression to increase dramatically.
Since cooler conditions can overwhelm conventional stabilizer liquid and stabilizer off gas compressors, the gasoline from the bottom of the stabilizers may not be stabilized. Gasoline is typically sent to atmospheric tanks. If the stabilization system is overwhelmed, the unstabilized gasoline is still sent to the storage tanks where the volatile components are allowed to evaporate to the atmosphere prior to sale. This is not only a loss of valuable product, but create high VOC emissions along with serious safety hazards since many of these components are heavier than air and can produce grade level clouds waiting for an ignition source.
The ethane, propane, butanes, and i-pentane that are condensed as part of the liquid are the target of the downstream NGL recovery unit in liquid form—the goal of NGL. Since these species are already condensed upstream of the NGL unit, they can be harvested at the stabilizer, stabilized to NGL specifications, and pumped to product. The harvesting of these species in the liquid stabilization feed treatment accomplishes several key goals.
Recycling these species as off gas from conventional stabilizers dramatically increases the size of the recompression required. Compressors are expensive to purchase, maintain, and operate. Harvesting these species at the stabilizer decreases the compressor size with the addition of minimal, inexpensive equipment.
Vapor loadings in the top section of the stabilizer are decreased since the off gas species are captured.
The downstream NGL plant capacity is increased due to upstream removal of NGL product in the stabilizer.
The gasoline in on-site storage tanks are at vapor pressure specification and do not pose a safety or environmental hazard.
The vapor side draw with the cooler and side stripper capture the NGL product that is being recycled to the downstream NGL plant. The reboiled side stripper allowed the operator to control the product specifications of the NGL that is condensed from the vapor side draw. The extended downcomer weir in the stabilizer provides the static head of liquid and the necessary back pressure to force the vapor to the side stripper. Both NGL and natural gasoline products are sent to storage. And minimal off gas is sent to the downstream NGL recovery unit, freeing up its capacity.
The application for the side stripped stabilizer is on the feed to NGL recovery plants where there is liquid that collects in the upstream feed surge drum—either because of induced mechanical refrigeration, ambient temperature causing condensation, or feedstock changes. Additionally, compression from well-head gas causes creates condensate that also must be fed to a stabilizer.
In the present invention, removal of the ethane, propane, butanes, and i-pentane is accomplished by targeting the vapor boiled off of the bottoms product at the reboiler using a vapor side-draw. A butane side-draw was utilized in the HF Alkylation processes in the Isostripper (UOP) or depropanizer.
In hydrofluoric acid processes, the vapor is drawn from a column at a point a few trays/stages above the reboiler. The bottoms product of the tower(s) is stabilized, high octane gasoline. The vapors that are stripped from the bottoms product are high in n-butane. These vapors are condensed and sent out as a blending product or to isomerization units. However, there is not a need to stabilize the n-butane.
In a natural gas liquids stabilizer, there are much lighter components that prevent the vapor from being a commercially viable product (Y-Grade). Therefore, these vapors need to be stabilized after they are condensed to typical NGL specifications (E.G.-methane <0.5 (LV) % of hydrocarbon, methane <1.5 (LV) % of ethane. Typical Y-grade composition (by weight for liquids, by volume for gases) is 3-20% ethane, 35-55% propane, 7-25% isobutane, 8-25% n-butane, 5-15% iso-pentane, 5-15% n-pentane, 10-50% hexane plus and 0-3% methane. The Y-grade composition is for liquefied petroleum gas.
The total vapor side-draw from the stabilizer feeds an air-cooled partial condenser. The cooled mixed phase feeds the side stripper where the quality of the bottoms product is controlled to specification by a reboiler. The stabilizer with a side stripper is shown in the drawings.
Since the total vapor side draw removes all of the vapor from the stabilizer, a total liquid trap tray with a tall downcomer is present on the stage above the vapor draw. The static head of clear liquid backup overflowing the weir provides the required pressure drop to force the vapor through the side draw cooler. Design margin is considered for the liquid static head above the pressure drop of the air cooler to ensure that vapors do not back up the downcomer. The partially condensed vapor from the side draw must be free draining to the side stripper to minimize pressure losses. The theoretical stages required for stabilization of the gasoline from the stabilizer and the NGL from the side stripper, these columns are short in length. And due to the free draining requirement from the air cooler to the side stripper, both tower bases and be close to grade. Only the air cooler is required to be elevated such that it free drains to the side stripper.
The side stripper overhead returns to the stabilizer above the liquid backup tray and provides vapor traffic to stripe light components in the liquid feed to the stabilizer. Additionally, the side stripper off gas contains some C2+ material that can be recovered in the liquid traffic going down the Stabilizer for another chance at harvesting in the stabilizer/side stripper system.
The off-gas from the stabilizer is recompressed and sent to the downstream NGL recovery unit, as it is with conventional stabilizers.
The stabilizer can be built in modular format along with the side stripper. Since gas compositions can change, different stabilizers can be fitted with different side stripper depending on the NGL and gasoline loadings.
Since the side stripper and additional tangent length required on the stabilizer for the side draw configuration is small relative to a large recycle compressor, the configuration is much more economical than conventional stabilizers. A detailed economic analysis still needs to be conducted. A relative perspective is that the stabilizer for scale of this study is 46 to 61 cm (18 to 24 inches) in diameter. Adding the side stripper configuration can reduce the recycle compressor requirements from the range of 500-1000 hp to about 80 hp.
The stabilizer net bottom has several uses that are potential for further development.
Since the stabilizer bottom is relatively hot, it must be cooled prior to sending to tankage. This heat can be recovered by using the hot product to reboil the side stripper as a heat medium. Very little duty is required to reboil the side stripper relative to the duty of the stabilizer. The reboiler outlet temperatures of the stabilizer and side stripper (204° C. (400° F.) and 54° C. (130° F.), respectively) provide ample approach temperatures to facilitate the heat integration.
Additionally, the stabilizer net bottom is excellent lean oil for absorption once it is cooled via air cooler. Utilizing this lean oil can be as simple as injecting (recycling) a small stream of it into the upstream feed surge drum to capture more propane and butane in the liquid feeding the stabilizer system. Or it can be used in a lean oil absorber tower to treat the feed to the unit in lieu of a feed surge drum.